[Oberon] HDMI question
skulski at pas.rochester.edu
Mon May 18 00:38:30 CEST 2020
For the record:
> Caps represent 0-ohm AC impedance so the impedance will be the same with or without caps but it doesn't matter.
The point is not the AC impedance of the caps, but whether or not the TMDS output transistors expect the DC level to work properly. I still remember the old school open emitter ECL or the new school open collector outputs. You cannot operate these topologies without DC connection to the power rail. Since your board works it must be somehow different from the ECL or OC, but it is clearly violating the specs. It works though it should not ;-)
> I'm not further expert in electrical standarts but my guess is that TMDS is current mode differential, not voltage mode > like LVDS so final termination in monitor may be 0ohm (theoretically)
LVDS is current mode. It steers 3.5 mA current into the far end termination, which is 100 ohms between the LVDS+ and LVDS-. This develops the nominal 350 mV voltage difference at the receiver. Unlike TMDS, there is no specified connection to either the ground or power.
The TMDS specifies 50 ohms from each TMDS+ and TMDS- to 3.3V. Differentially it is still 100 ohms, but the middle point of the termination is supposed to be centered at 3.3V. This topology resembles the open collector, but it is differential. I bet that the monitor is providing these resistors. It cannot provide 0 ohms because this would be a short.
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